This invention relates to a cylindrical filter having a novel structure.
As to currently commercially available cylindrical filters, various kinds have been known such as those obtained by winding a cloth or a non-woven fabric in the form of rolls, followed by adhering the resulting rolls by heat or with a resin, those obtained by winding a fiber bundle of cotton yarns, multifilaments, etc. on a core material such as a perforated cylinder, etc., hereinafter referred to as a winding-type filters, and among these, those using multifilaments have been used in many cases in the aspect of convenience of production process.
In the case of the winding type filters, in order to impart a required mechanical strength thereto, the fiber bundle should be wound up under a certain tension so that it is difficult to obtain those having a broad void between fibers. Particularly in the case where the fiber bundle is of multifilaments, since the respective single filaments constituting the multifilaments are made parallel, not only the void between single filaments but also that between the fiber bundles are liable to become narrow. Thus, when filters produced from such a fiber bundle are used, there is a drawback that solid matters are liable to be caught only at parts close to the surface so that resistance to water pass rises in a relatively short time, that is, the so-called filtration life is short. In order to avoid such a drawback, it may be possible to make the outer layer of filters rougher and make the inner layer thereof denser, by means of, e.g., winding up the fiber bundle gradually or stepwisely reducing the tension from the beginning of the winding toward the end thereof, or using coarse fibers on the outer layer of filters, but there is another drawback that the steps are complicated.